An impact wrench is configured to tighten bolts, nuts, and the like by applying impulsive force generated by a rotationally driven hammer to an anvil serving as an output shaft. An impact wrench is provided with a motor, a spindle, a hammer, and an anvil as its main components. The operation will now be described briefly below.
The motor causes the spindle to rotate at a predetermined number of revolutions, and the rotational force of the spindle is transmitted to the hammer. Rotation of the hammer causes claws provided on the hammer to strike engaging claws provided on the anvil. Then, this impact provides a predetermined torque to a socket attached at the tip of the anvil, thus tightening bolts, nuts, and the like.
Such impact wrenches as described above can be classified into several groups according to their different mechanisms that deliver a rotary impact. A typical impact wrench is one whose rotary impact mechanism is configured by cam grooves that are formed in the spindle and the hammer, steel balls sandwiched between the cam grooves, and a spring that biases the hammer in the direction of the anvil (see, for example, Patent Document 1).
In the impact wrench described in Patent Document 1, the hammer, in principle, moves in the direction of the axis of the spindle while rotating. Consequently, an impulse is applied in the axial direction in addition to an impulse that causes rotation of bolts, nuts, and the like, and these impulses cause vibrations in a direction orthogonal to the axis of the spindle and in the direction of the axis of the spindle. The vibrations cause fatigue to the worker and hence reduced operation efficiency as well as numbness in the hand, and therefore it is desired to mitigate the vibrations.
The result of a measurement of the vibrations indicates that the magnitude of vibrations in the direction of the axis of the spindle is about three times that of vibrations in the direction orthogonal to the axis of the spindle, and it is effective to reduce the vibrations in the direction of the axis of the spindle in mitigating vibrations.
As means for mitigating the vibrations in the direction of the axis of the impact wrench, it is conceivable to decrease the mass of a primary hammer, which is the source of the vibrations in the axial direction, by providing a secondary hammer that only contributes to transmission of an impulse in the rotational direction, separately from the primary hammer. However, no specific proposal has been made.
Although the purpose is different from the mitigation of the vibrations in the axial direction, it has been proposed to provide a secondary hammer that only contributes to transmission of an impulse in the rotational direction in order to regulate the rotary impact force of the hammers, and engage or disengage the secondary hammer with or from the primary hammer (see, for example, Patent Document 1, Patent Document 2).